Currently, energy is largely dependent on fossil fuels such as petroleum, coal and natural gas, but the future depletion of fossil fuels is a serious concern. Also, when energy is obtained from fossil fuels, carbon dioxide discharge inevitably occurs. Such discharge imposes a large burden on the environment, and is thus regarded as a major problem.
Recently, owing to such concerns, more attention is being paid to photovoltaic power generation, with active development work currently being carried out on, for example, silicon solar cells that use crystalline (single-crystal or polycrystalline) silicon or amorphous silicon, and on compound semiconductor solar cells which use gallium and arsenic. However, because production costs and other problems remain to be overcome, photovoltaic power generation still lacks widespread applicability. At the same time, numerous disclosures have been made concerning solar cells which use photosensitizing dyes, but such dyes have had a low conversion efficiency and a poor durability.
This was the situation in the art when Gratzel et al. published a report in 1991 (Non-Patent Document 1) describing a photovoltaic electrode and a photovoltaic cell that use an inorganic semiconductor porous material photosensitized with a dye. The photovoltaic device in their report is manufactured using a relatively inexpensive inorganic oxide semiconductor such as titanium oxide, and enables photovoltaic devices to be obtained at a lower cost than general-purpose silicon solar cells. However, it is not currently possible to achieve a high photovoltaic conversion efficiency without the use of a ruthenium-based sensitizing dye. Owing to the high cost of such dyes and the low crystal abundance of ruthenium, significant challenges remain to be overcome before a stable supply of such photovoltaic devices can be established. Active development efforts are also being carried out on organic dyes, although these have not yet reached a practical stage owing in part to their low conversion efficiency. Dyes with specific acrylic acid sites and sensitizing dyes having amide derivatives have also been developed (see Patent Documents 1, 2 and 3), but fall short in terms of performance.    Patent Document 1: JP 2002-011213 A    Patent Document 2: JP 2004-143355 A    Patent Document 3: JP 2005-097561 A    Non-Patent Document 1: Brian O'Regan and Michael Gratzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films,” Nature 353, 737-740 (October 1991; Great Britain).